An A-D converter that utilizes V-F (Voltage-Frequency) conversion is disclosed in the following Non patent document 1. In this type of A-D converter, the frequency of a pulse signal that is output from the V-F conversion unit fluctuates according to the magnitude of the input voltage, which is an analog signal, and the A-D converter generates a digital signal by counting the number of pulses contained in that pulse signal with a counter.
Non patent document 1: ANALOG DEVICES AN-277 APPLICATION NOTE, III INSTRUMENTATION APPLICATIONS, Analog-to-Digital Conversion, FIG. 9, Internet<URL:http://www.analog.com/UploadedFiles/Application_Notes/511072672AN 277.pdf>